text: MHD thrusters are classified in two categories according to the way the electromagnetic fields operate:
  * Conduction devices when a direct current flows in the fluid due to an applied voltage between pairs of electrodes, the magnetic field being steady.
  * Induction devices when alternating currents are induced by a rapidly varying magnetic field, as eddy currents. No electrodes are required in this case.
  As induction MHD accelerators are electrodeless, they do not exhibit the common issues related to conduction systems (especially Joule heating, bubbles and redox from electrolysis) but need much more intense peak magnetic fields to operate. Since one of the biggest issues with such thrusters is the limited energy available on-board, induction MHD drives have not been developed out of the laboratory.
  Both systems can put the working fluid in motion according to two main designs:
  * Internal flow when the fluid is accelerated within and propelled back out of a nozzle of tubular or ring-shaped cross-section, the MHD interaction being concentrated within the pipe (similarly to rocket or jet engines).
  * External flow when the fluid is accelerated around the whole wetted area of the vehicle, the electromagnetic fields extending around the body of the vehicle. The propulsion force results from the pressure distribution on the shell (as lift on a wing, or how ciliate microorganisms such as Paramecium move water around them).
  Internal flow systems concentrate the MHD interaction in a limited volume, preserving stealth characteristics. External field systems on the contrary have the ability to act on a very large expanse of surrounding water volume with higher efficiency and the ability to decrease drag, increasing the efficiency even further.